Melt butter in a medium kettle and add chopped turnips and onions. Saute over medium heat for about 5 minutes or until onions are translucent. Stir frequently. Bring 6 to 8 cups water to the boil--the amount depending on how much soup you wish to make, as well as its desired strength. Pour the boiling water into the turnip/onion mixture. Break the bread slices into the kettle. Heat the soup to the boil; reduce heat and simmer for aout 15 minutes or until turnips are tender when pierced with a fork. Set the soup aside to cool. Season with salt, if desired, and pepper. When the soup has cooled sufficiently, four into a food processor or blender or press through a sieve with a wooden spoon. Note: At this point the soup can be refrigerated for a day or two, or frozen for several weeks. Later the egg, cream and garnish will be added. Before serving, reheat the soup--but do not boil--and add the egg yolks beaten with the cream. Stir constantly until the mixture is blended and creamy. Check seasoning. Serve in heated soup bowls and garnish with parsley or chives.